This invention relates generally to methods and apparatus for detection of radiation, and, more particularly, relates to radiation detectors useful in bone densitometry, for generating data representative of ionizing radiation absorption along a plurality of ray paths.
Bone densitometry is a technique for measuring the bone mineral content (BMC) of bones to detect bone loss and thereby diagnose certain disease states. Typically, bone densitometry systems employ an ionizing radiation source--such as an X-ray or radioisotope source--and one or more detector elements. The portion of a patient's body to be evaluated is placed between the source and the detector elements. When rays generated by the source pass through the body and impinge upon the detector elements, the detectors produce electrical signals representative of the density and amount of bone material in the region between the source and the detectors. Systems of this sort are well known in the art. See, for example, Cameron and Sorenson, Science, Vol. 142, p. 230 (1963) and Osteoporosis Update 1987, H. K. Genant, ed.
Conventional bone densitometry systems, however, suffer from a number of deficiencies. In most bone densitometry systems, a single source and single detector are employed. As a result, only a small fraction of the source output is intercepted by the detector, and a large, cumbersome and expensive X-ray tube or radioisotope source is required to effect rapid and precise measurements.
It is thus an object of the invention to provide detection methods and apparatus which can provide rapid, precise radiation detection without attendant bulk, complexity, and expense.
Certain conventional bone densitometry systems implement area measurements by mechanically scanning the source and detector in a raster pattern over the region to be measured. Motion of the patient during this time results in a blurred and imprecise image. It has also been proposed in prior art to replace the single detector with a line of detectors and thus to measure a single line in the body simultaneously. Measurement over the area is then obtained by motion in a single axis, by translating the detection line across the area to be scanned. This technique increases scanning speed, thereby decreasing patient motion effects. Additionally, the portion of source output which is intercepted by the detectors is greatly increased, so that the total output and attendant cost of the source can be reduced without increasing scan time. However, certain line detectors typical of the prior art utilize expensive solid state silicon or germanium detector arrays which are not generally available commercially. Other such systems employ scintillation crystals, such as sodium iodide, coupled to relatively complex and expensive photomultiplier tubes or photodiodes.
It is accordingly a further object of the invention to provide a linear radiation detector which is simple and inexpensive to construct and employ.
Another configuration of bone densitometry system utilizes multiple ionization detector chambers constructed by placing conductive electrodes in an ionizable gas medium. Such arrays have been employed in the detection of radiation for X-ray tomographic systems Devices of this type are disclosed in U.S. Pat. Nos. 4,031,396 and 4,129,783. The precision of these systems, however, is significantly degraded by physical disturbances such as mechanical vibration. In addition, these systems have no provision for determining the energy spectrum or distribution of the detected X-rays, a critical step in accurate determination of BMC values.
It is therefore an object of the invention to provide radiation detection methods and apparatus which generate high Precision measurements of ionizing radiation, with relative insensitivity to mechanical vibration.
It is yet another object of the invention to provide such methods and apparatus which facilitate determination of the energy distribution of the radiation.